1. Field of the Invention
The present invention relates to light-emitting elements using electroluminescence. In addition, the present invention relates to light-emitting devices and electronic devices including the light-emitting elements.
2. Description of the Related Art
In recent years, extensive research and development on the light-emitting elements using electroluminescence have been carried out. In a basic structure of the light-emitting elements, a light-emitting substance is interposed between a pair of electrodes. When voltage is applied to the elements, the light-emitting substance can emit light.
Since the above light-emitting element is of a self-light-emitting type, it has advantages that the visibility of its pixels is higher than that of a liquid crystal display, that a backlight is not required, and the like. Because of the advantages, the light-emitting element is known to be suitable for a flat panel display element. Another major advantage of the light-emitting element is that it can be manufactured to be thin and lightweight. In addition, the light-emitting element has a feature that its response speed is extremely high.
The light-emitting element can be formed into a film form. When the element is formed to have a large area, plane emission can be easily obtained. This feature is hard to obtain from point sources typified by an incandescent lamp and an LED, or linear sources typified by a fluorescent light. Thus, the light-emitting element is of value as a plane light source that can be applied to lighting and the like.
Light-emitting elements using electroluminescence are broadly classified depending on whether the light-emitting substance is an organic compound or an inorganic compound.
In a case where an organic compound is used as the light-emitting substance, by the application of voltage to the light-emitting element, electrons and holes are injected from a pair of electrodes into a layer containing the light-emitting organic compound, and current flows. When these carriers (electrons and holes) are recombined, the light-emitting organic compound is excited. When the excited state returns to a ground state, light emission is obtained. Owing to the above mechanism, such a light-emitting element is referred to as a current-excitation light-emitting element.
Note that types of excited states of an organic compound include a singlet excited state and a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state, phosphorescence.
Problems of such a light-emitting element mainly originate from unsatisfactory performance of materials. In order to enhance the element characteristic, structures of the light-emitting element and materials used therein have been developed.
For example, according to Non-Patent Document 1, a method called Triplet Harvesting was used to form a highly efficient light-emitting element.
[Reference]
[Non-Patent Document]
    [Non-Patent Document 1]
M.E. Kondakova, et al., SID 08 DIGEST, pp. 219-222 (2008)
In a structure described in Non-Patent Document 1, a light-emitting layer (Yellow LEL) containing a yellow emissive phosphorescent compound is provided on a cathode side of a light-emitting layer (Blue LEL) containing a blue emissive fluorescent compound. Therefore, a part of the triplet excitation energy of the blue emissive fluorescent compound is transferred to the cathode side, which allows the yellow emissive phosphorescent compound in the Yellow LEL to emit light. On the other hand, since an electron-blocking layer (EBL) having greater triplet-excitation energy than that in the Blue LEL is provided on the anode side of the Blue LEL, the transfer of the triplet excitation energy of the blue emissive fluorescent compound to an anode side is impossible. Thus, a part of the triplet excitation energy of the blue emissive fluorescent compound is consumed through the nonradiative process and does not contribute to the light emission.
In view of the above problem, an object of one embodiment in the present invention is to enhance the luminous efficiency of a light-emitting element.
Another object is to reduce the power consumption of a light-emitting element, a light-emitting device, and an electronic device.